In an electric power generation system, high purity feedwater is heated in a boiler to create steam which is then expanded through a steam turbine. The turbine shaft is connected to an electric generator shaft which, when rotated, creates electric energy. The steam that exits the turbine is condensed in a heat exchanger, thereby creating a vacuum. The difference between the steam pressure and the vacuum created is the driving force of the steam through the turbine. The condensed steam may be purified and preheated and is then directed back to the boiler as feedwater, completing the power cycle.
Many electric power generating plants purify condensate to remove contamination, particularly ionic materials from a raw water supply, that may enter the feedwater, steam, or condensate during the power cycle. Such purification is effected by the use of demineralizers to purify the condensate through an ion exchange technique. Two types of demineralizers are used for condensate purification: (i) deep bed demineralizers and (ii) filter demineralizers.
Deep bed demineralizers utilize electrostatically charged resin beads to remove dissolved ions in the condensate. Specifically, the condensate is passed through a bed of resin beads which are retained in a demineralizer vessel. The deep bed demineralizers typically have an effective pore rating on the order of 40-50 microns and are only marginally useful in removing particulates from the condensate.
Filter demineralizers utilize powdered ion exchange resins and/or inert filter aids such as cellulosic fibers which are precoated onto fine porous elements. The porous elements typically include spirally welded metal elements, powdered metal elements, wedge wire elements, and yarn or string wound elements. The condensate is passed through the precoated elements, and the precoated elements remove dissolved contaminates and trap particles. The precoats on the filter demineralizers typically have an effective pore rating on the order of 5-30 microns, with the underlying filter media having a pore rating of 5-120 microns. The filter demineralizers have an overall effective pore rating on the order of 5-30 microns and are therefore somewhat more effective in removing particulates from the condensate as compared to deep bed demineralizers. Solids levels exceeding the relatively low levels of steady-state condensate, e.g., the high solids levels in condensate during start-up and during flow transients associated with load cycling operations, however, lead to the need for extensive backwashing of the precoat resin, with an associated high cost of operation of the filter demineralizers.
The contaminates in feedwater, steam, and condensate in a power generating plant typically must be maintained at a level of no greater than about 50-250 ppb total suspended solids, most typically no more than about 50 ppb total suspended solids. During steady-state operation, the impurities in the condensate are low, and the water quality is typically maintained within the requisite levels through use of deep bed or filter demineralizers. When a unit, particularly a fossil-fueled unit, is shut down for maintenance or for other reasons, however, air enters the equipment previously flooded with water, and iron oxides and other corrosion products form on the carbon steel surfaces of the power generation equipment. Upon plant start-up, the oxide contamination is swept into the boiler feedwater which typically contains at least about 500 ppb total suspended solids, most typically at least about 1000 ppb total suspended solids. The contamination in the feedwater during boiler operation is left behind by the steam and forms into scale on the boiler tubes. This scale reduces boiler efficiency and can eventually lead to boiler tube failure. For that reason, boiler equipment guarantees usually mandate maximum contaminant levels in feedwater prior to boiler operation.
Power generation plants, and, in particular, older fossil-fueled power generation plants, typically have difficulty purifying the condensate to meet requisite levels after unit outages and prior to unit start-up. Plants with condensate purification systems with deep bed demineralizers engage in extended recirculation of the condensate after outages through the condensate purification systems. Since the deep bed demineralizers are not designed to remove particulates, and are therefore inefficient at doing so, extensive recirculation of the condensate is necessary to reduce particulates to the requisite level. The time necessary for recirculation of the condensate to meet the requisite total suspended solids levels typically ranges from about 12 to about 48 hours prior to actual plant start-up, although recirculation times may be as much as about 96 hours.
The use of a filter demineralizer provides for the reduction of particulates in the condensate to the requisite level in about 8 hours, during which time the plant is gradually brought fully on-line. Although only one pass of the condensate is required through a filter demineralizer during start-up, the filter demineralizer is not designed to operate in the high particulate loading environment characteristic of plant start-up, and, therefore, the filter demineralizer frequently clogs during plant start-up. Each time the filter demineralizer becomes clogged, the filter demineralizer must be backwashed, and the precoat must be replaced.
Those plants without any condensate purification system operate the boiler at reduced levels during start-up and attempt to purify the feedwater by blowing down high solids containing water from the boiler steam drum for about 8-10 hours and replacing it with high purity make-up water. In all cases, considerable generating capacity and potential revenue is lost by the additional down-time required to treat the condensate to reduce the contaminant level to the requisite level prior to bringing the plant fully on-line.
There remains a need, therefore, for a method of treating condensate from a power generating plant, particularly a fossil-fueled power generating plant, which can efficiently and economically reduce the contaminant level of the condensate to the requisite level upon start-up after a shut-down period. It is an object of the present invention to provide such a treatment method.
It is another object of the present invention to provide a treatment method which can also be used to continuously treat condensate from a power generating plant during steady-state operation to ensure that the contaminant level of the condensate remains within an acceptably low level.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.